In recent years, fine particles in the order of nanometers have been applied to various devices. For example, fine metal particles of nickel are used for a ceramic capacitor at present, and fine particles with a particle diameter of 200 nanometers or less having good dispersibility are considered to be used for a next generation ceramic capacitor.
Furthermore, fine particles of silicon monoxide (SiOx: x=1 to 1.6) having a lower oxygen content than silicon dioxide are utilized as an antireflection film of an optical lens or a deposition material of a gas-barrier film for food packaging. Recently, application of fine particles to an anode material of a lithium-ion secondary battery is also expected.
Some common methods of producing fine particles in the order of nanometers are: a method of introducing a bulk material as a raw material together with beads such as, ceramic beads and zirconia beads, and miniaturizing the material into particles by mechanical crushing; a method of melting and evaporating a material and spraying the material to air or water to obtain fine particles; and a method of chemically obtaining fine particles by electrolysis or reduction and so on. Among them, a method of producing fine particles in a vapor by using thermal plasma (approximately 10000° C.) such as high-frequency discharge and DC or AC arc discharge is extremely useful from the perspective of producing fine particles with excellent dispersibility, less contamination and composite fine particles formed of plural kinds of materials can be easily composed.
FIG. 4 shows a schematic cross-sectional view of a production apparatus for fine particles utilizing thermal plasma disclosed in JP-A-2004-263257 (PTL 1).
The apparatus is configured to include a material supply device 110 and plural electrodes for generating arc discharge in a reaction chamber 101, a fine particle collection section 103 for collecting fine particles 118, a gas supply pipe (not shown) for supplying gas to the reaction chamber 101, a valve for regulating pressure and a pump 113 for exhausting the gas. After an argon gas is introduced into the reaction chamber 101 from a gas supply pipe and the pressure is regulated, AC power is applied to plural electrodes 104 from plural AC power sources 105 to thereby generate arc discharge 116 as thermal plasma. Material particles 117 in the material supply device 110 are introduced to the generated arc discharge 116 from a vertically downward direction together with a carrier gas. The introduced material particles 117 are evaporated and vaporized by the arc discharge 116, then, rapidly cooled and coagulated in an upper part of the reaction chamber 101 to thereby generate the fine particles 118. The generated fine particles 118 are introduced into the fine particle collection section 103 along the gas flow inside the reaction chamber 101 and collected by a filter inside the fine particle collection section 103.
FIG. 5 shows schematic views of a discharge state in which multi-phase AC arc plasma used in PTL 1 is observed by a high-speed camera at a certain moment. Black triangle marks shown in FIG. 5 represent tip ends 120 of the electrodes 104 shown in FIG. 4 of PTL 1, and six electrodes 104 are arranged radially at intervals of 60 degrees. AC power is applied to the electrodes 104, namely, an electrode E1 to an electrode E6 respectively so that phases of which are shifted by 60 degrees, thereby generating the arc discharge 116 in a planer direction. An arc discharge region 121 at a certain moment shown in FIG. 5 represents a high-temperature region of 5000 degrees or more as a result of measuring a gas temperature by spectroscopic analysis. As shown in FIG. 5 (upper figure), a discharge D1 is generated from the tip end 120 of an electrode E1 at a certain moment, a discharge D4 is generated from the tip end 120 of an electrode E4, a discharge D2 is generated from an electrode E2 at a next moment (see FIG. 5 (middle figure)) and a discharge D5 is generated from an electrode E5 sequentially. The arc discharges D1 to D6 are sequentially generated clockwise from the electrode E1 to the electrode E6, and discharges are generated to the electrode E1 repeatedly. At that time, the arc discharge D1 itself overlaps with discharges from other electrodes 104 while swinging from the left side to the right side with respect to the electrode 104 to increase the discharge region 121, then, disappears at a next moment. The discharges from the electrode E1 to the electrode E6 are driven at 60 Hz as AC power and the arc discharge is repeatedly generated at 16.7 ms as one cycle.
However, there are problems such as a material not processed at all (unprocessed) or a material in which the process ends in a melting stage as the temperature is insufficient to be evaporated are generated only by the discharges from the electrode E1 to the electrode E6, which leads to low processing efficiency.